In a two-node bi-directional Free Space Optical (FSO) communication system, the two FSO nodes exchange data encoded on optical carrier beams sent across an unobstructed line of sight (LOS) between the two nodes. As shown in FIG. 1, a conventional two-node bi-directional system is illustrated. As shown, a first node 2 and a second node 3 communicate by transmitting and receiving a signal 6, 7 sent between the nodes. The data can be encoded on the signals in any manner; a binary, on-off, exemplary signal is illustrated for simplicity. Each node has an optical output 4 for transmitting the desired signal 6, 7, and also an optical input 5 for receiving the transmitted signal. Once received, the internal electronics of the node can decode the signal and obtain the transmitted data.
The communication system only works if the transmit path of the first node is aligned with the receiving components of the second node. In order to optimize tracking, conventional systems have split the received beam into two paths: one for detection and one for alignment. FSO systems may also integrate the transmit and receive paths into a single aperture device. However, this may increase system complexity as it requires additional splitting of the beam from transmit source to receive detector. Also, the integration of beam paths invites misalignment that may adversely affect long range communication.
Traditionally, FSO systems include multiple optical components, including beam splitters and corresponding optics to be able to detect the beam angle and align the beam, or perform other functions. For example along the receive path, receive optics are included to detect the angle of the received beam as well as receive data from the received beam. Conventionally, the receive and transmit optical components (i.e. detector/source) are fixed relative to the node. Internal optics are then used to finely align the beam on the fixed optics. In an exemplary system, a fast steering mirror (FSM) may be used to position the beam in the desired orientation/alignment.
To provide the fine tuning in a fast and precise platform, therefore requires substantially more optics and system complexity. This introduces additional alignment errors and potential for drift that continually needs to be adjusted. Therefore, exemplary systems are large, costly, and inefficient for long term use.
The other alternative would be to move the terminal optic itself. However, given the weight, complexity, connectivity, and configuration of these optics, they generally cannot be moved easily, quickly, and precisely. The FSM therefore provides the faster and more precise response. Accordingly, conventional applications contemplate fixed terminal components, such as receive detector, and transmit source, while alignment and other path manipulation is performed by intervening optics.